The original implementation of Raman spectroscopy utilized a scanning monochromator; the Raman scattered light was passed through a slit, the width of which contributes to the frequency resolution, onto a diffraction grating, which bent the remaining light to various degrees corresponding to wavelength. The exit slit of the monochromator selected a narrow band of wavelengths to be measured by the single element detector. The grating was then rotated or the slit translated in order to trace out the Raman spectrum on the detector as a function of time.
Another implementation is Fourier-Transform (FT) Raman spectroscopy, in which Raman scattered light is collimated and passed into an interferometer and then onto a single element detector. Frequency resolution can be obtained by mechanically translating one mirror of the interferometer, which results in an interferogram traced out on the detector as a function of time. The interferogram can be Fourier transformed to obtain the Raman spectrum. Advantages of FT-Raman systems include the multiplex, or Fellgett, advantage and optical throughput, or Jacquinot, advantage. The multiplex advantage is obtained because all or nearly all of the Raman scattered light from the sample is captured by the detector. This can be compared to the scanning monochromator case, in which only one wavelength or narrow band of wavelengths is measured at a time. The optical throughput advantage is obtained because no slits or gratings are required for operation, which would otherwise limit the number of photons reaching the detector.
The other advantage of FT-Raman systems is that they can utilize a 1064 nm laser for excitation, which can greatly reduce the amount of fluorescence background measured from many samples. However, FT systems are bulky, costly, and typically not portable due to mechanical stability requirements. FT systems are rarely, if ever, used for in vivo clinical applications.
The advent of the charge-coupled-device (CCD) detector allowed for “dispersive” Raman spectroscopy. Dispersive Raman systems have the same front end as systems utilizing a scanning monochromator; Raman scattered light is collected and passed through a slit, the width of which can determine the frequency resolution, and then onto a diffraction grating. However, rather than use an exit slit and a single element detector to trace out the Raman spectrum as a function of time, dispersive systems utilize 1-D or 2-D detector arrays or CCDs to image the entire Raman spectrum at once. Dispersive systems have a multichannel advantage in that all of the wavelengths are measured in parallel, or simultaneously, as opposed to the serial, or sequential, measurements of the other implementations. However, dispersive systems have limited optical throughput due to the use of an entrance slit and grating.